Recently, adverse effects on the human body of ultraviolet rays included in sunlight have become widely known, and various measures directed toward alleviating or preventing the adverse effects have been considered.
Ultraviolet rays are divided into three categories based on dermatological effects, i.e., long-wavelength ultraviolet rays (UV-A) (320-400 nm), medium-wavelength ultraviolet rays (UV-B) (290-320 nm), and short-wavelength ultraviolet rays (UV-C) (shorter than 290 nm).
Of these, UV-C can cause fatal harm to living things, including human beings. So far, UV-C has been absorbed by the ozone layer of the upper atmosphere before reaching the earth (However, there is concern that recent depletion of the ozone layer may allow UV-C to damage living things on the earth).
UV-A and UV-B reach the earth, and human beings are exposed to them. UV-A and UV-B have a number of effects on the human body.
UV-A and UV-B have many more adverse effects rather than beneficial effects, such as promotion of production of vitamin D in the body.
The development of agents for preventing harm caused by UV-B started earlier and was carried out more actively than that for preventing harm caused by UV-A. However, adverse effects caused by UV-A, such as accelerated aging of the skin, have been noted recently, and a number of measures to effectively screen UV-A have been proposed.
Presently, when an inorganic substance is used to screen ultraviolet rays in sunlight, a metal oxide having an excellent UV-screening effect such as titanium dioxide or zinc oxide is incorporated into a composition for external use, such as a cosmetic composition, to thereby make use of the UV-screening effect of the metal oxide for protection of the human body from UV rays in sunlight. However, when the metal oxide is used to improve a screening effect against UV-A, whose wavelength range includes a wavelength proximate to the wavelengths of visible light, there arises a problem that the application area of the skin appears unnaturally white due to a characteristic of metal oxides scattering visible light. Therefore, the content of the metal oxide incorporated into a composition for external use is limited. As a result, a composition for external use has not yet led to exerting a sufficient screening effect against UV-A.
At present, measures for screening UV-A have not been fully established.
In view of the foregoing, an object of the present invention is to provide a metal oxide which enables effective UV-screening, i.e., which has excellent screening ability against UV rays, particularly UV-A, as well as excellent transmission with respect to visible light. Another object of the present invention is to provide a UV-screening composition that contains the metal oxide and can be applied to external use.